This invention relates to the slicing of crystals and other materials into wafers and, more particularly, to systems for slicing hard and expensive materials with wire blades.
To slice wafers from a workpiece, such as an ingot of gallium arsenide, silicon, sapphire, silicon carbide, cadmium telluride, or germanium, it is desirable to provide a radial cut profile between the workpiece and one or more slicing blades, to reduce the length of the cutting blades in contact with the workpiece, and to provide for a relatively constant feed force between the blades and the workpiece.
U.S. Pat. Nos. 4,646,710 and 4,727,852, which are expressly incorporated herein by reference, disclose such slicing systems. In one embodiment, a wafering machine has taut wire blades strung on a bladehead that reciprocally moves the blades over a workpiece. The workpiece is supported by a holder that rocks about an axis perpendicular to the wire blades at a frequency that is less than the frequency of reciprocation of the bladehead. Guide rollers are placed near, and on opposite sides of, the workpiece to reduce the unsupported lengths of the blades. Slicing effectiveness can increase rapidly when there is a decrease in the contact length between the workpiece and the wire blade. Decreasing this contact length while maintaining a constant feed force causes the pressure between the blade and the workpiece to increase, thereby causing more effective cutting.
Generally speaking, it is desirable to slice such expensive material with wire because the kerf width of wire is small, thus resulting in low levels of material waste. Wire can be used with either loose abrasive or abrasive fixed on the wire. Loose abrasive slicing, which involves abrasive tumbling between the wire and the workpiece causing material removal by an indentation process, is also known as a 3-body abrasion because it involves three bodies: the abrasive, the wire, and the workpiece. In the case of fixed abrasive slicing, which is also known as 2-body abrasion, the fixed abrasive particles are forced into the workpiece to plow out the material. When fixed abrasive slicing is used, it is sufficient to attach the abrasive to the cutting edge of the wire.
U.S. Pat. No. 4,384,564, discloses a method of electroplating on the cutting edge of the wire to reduce the kerf width, and U.S. Pat. No. 5,438,973 discloses reducing the kerf width further by using a tear-drop shaped wire. These two patents are also expressly incorporated herein by reference. When loose abrasive slicing is used, wire wear and degradation of the abrasive occurs as the slurry carrying the abrasive becomes contaminated with the kerf and the abrasive breaks down, resulting in high expendable costs for the wire and abrasive.
It is known to cut a workpiece while it is rotating. In U.S. Pat. No. 5,351,446, for example, an annular, inner diameter saw is rotated while an ingot is also rotated to cut until a small central portion is left. A continuous loop wire saw is then used to cut a central portion while the ingot is again rotated. A nub left at the center is then ground down.
In U.S. Pat. No. 5,878,737, an apparatus for fixed abrasive slicing of a workpiece uses a continuous wire that cuts a workpiece that is rotated about its longitudinal axis. This rotation is provided through the use of the engagement of collet fixtures and drive rollers to provide a tangential rotational force. With a continuous wire, high speed can generally be achieved, and such rotation is used to minimize contact length.
In U.S. Pat. No. 5,564,409, a long wire that runs between a feed spool and a take-up spool is used to cut a workpiece using a loose abrasive slurry. The workpiece has an opening in the center. The workpiece is rotated by rotating a shaft and sleeve that extend through the workpiece. Workpieces with such openings may be used for disk drives; workpieces without such openings are used, for example, for semiconductor wafers.
The present invention features an improved slicing machine that provides a high relative speed between a wire blade and a workpiece surface, while a short contact length is maintained by rotating the workpiece at a high rotation speed, up to 15,000 rpm depending on the diameter of the workpiece and the ability to remove heat. In an exemplary embodiment, the relative speed at the work surface (i.e., surface speed for a 4 inch diameter workpiece) is 13 m/sec at 2,500 rpm, and 26 m/sec at 5,000 rpm. Thus, cutting rates can be achieved that exceed previous cutting rates achieved with rocking by a factor of 50, and that exceed multiwire slurry slicing rates by a factor of 10 or more. In addition, high wafer accuracy can be achieved with low surface damage. The depth of penetration and chip size can be controlled by determining the rotation rate and the infeed rate. For example, when the infeed rate is held constant and the rotation rate is increased, the depth of penetration and chip size per revolution are decreased. The chip size determines the surface finish and depth of damage.
The workpiece is preferably held by attaching spacers against axial ends of the workpiece and rotating the workpiece and the spacers with a motor. By mounting the ends of the workpiece to spacers with wax or epoxy, the workpiece can be balanced such that it can be rotated at high speed without vibration.
The cutting is preferably achieved in at least two stages. In a first stage, the workpiece is rotated continuously for many rotations at a time. These rotations can be all in one direction, or the device can cause the rotation to periodically reverse direction. A second stage is then undertaken after the diameter of the uncut core portion has been reduced to a smaller amount. When the uncut section is reduced to a smaller diameter, there is a risk that the remaining center portion may break. The specific diameter that triggers the change from the first to second stage depends on factors such as the operator""s tolerance to risk, experience, and the type of material being cut. For sapphire, for example, it is believed that second stage cutting should begin with about one-quarter inch (6-7 mm) of a central core remaining.
In the second stage, the workpiece is rotated back and forth, preferably through angles less than 360xc2x0, and could be lower, such as 20xc2x0. In other words, the workpiece oscillates back and forth until the remaining center portion is cut.
Between the first and second stage, a holder is preferably attached to the workpiece to hold the partially cut wafers or substrates while the remaining core is cut.
In the first stage the workpiece can be rotated in one direction for some period and then rotated in the reverse direction to expose the opposite edge of the diamond abrasive for improved cutting efficiency, wire life, or to achieve more uniform kerf to compensate for wear of abrasive, but preferably these alternation are at least one full rotation in each direction.
The present invention further allows for cutting of multiple workpieces with a series of parallel wire blades with the workpieces side-by-side; or workpieces can be connected to a rotating spindle and cut together.
The wire that is used can have diamond or some other abrasive on a cutting side or all around. If all around and if the blade is part of a bladehead, the bladehead is designed in a symmetric manner so that it can easily be turned over to make the wear more even. This method for cutting through whole material can also be used with the loose abrasive method.
The present invention thus provides improved cutting with less risk of breakage, particularly with the two stages of cutting, and also by using a support bar for holding the partially cut pieces. Other features and advantages will become apparent from the following description, including the drawings, and from the claims.